1. Field of the Invention
This invention relates to an aryl ester compound which can be used mainly as a curing agent for an epoxy resin and can give a cured product having a low dielectric constant and a low dielectric loss tangent; an epoxy resin composition using the same, and a laminate prepared using the epoxy resin composition. These are suitable for a molding material, a paint and varnish, a coating material, a civil engineering material, a constructional material and in particular, in the electric or electronic uses which work at high-frequency region, for example, a resin for laminate or an IC-sealing resin.
2. Description of the Prior Art
Among the epoxy resins used in the electric and electronic uses, the resin for a printed wiring board has heretofore been mainly a combination of a bisphenol type epoxy resin with a dicyandiamide. In recent years, the resin has been required to have a low dielectric constant for the purpose of the improvement of signal velocity and the impedance-matching of circuit as the printed wiring board has been made thinner and more multiple in layer and also required to have a low dielectric loss tangent for the purpose of diminishing the signal transmission loss as a high-frequency signal is used. For these purposes, a method of combining a conventional epoxy resin with a thermoplastic resin having a low dielectric constant and a low dielectric loss tangent has been devised. For example, a method of modifying the epoxy resin with a reactive polybutadiene resin and a method of dispersing a polytetrafluoroethylene resin powder have been proposed.
However, in these prior techniques, the epoxy resin which is the basic material has a high dielectric constant, and hence, the proportion of the thermoplastic resin to be combined becomes large in order to achieve the desired dielectric constant, and hence, the heat resistance, adhesiveness, dimension stability, chemical resistance and the like which are characteristic features of the epoxy resin are damaged. Therefore, there have been earnestly desired such an epoxy resin curing agent that a cured product having a low dielectric constant and a low dielectric loss tangent can be obtained without damaging the heat resistance, adhesiveness and processibility of an epoxy resin and an epoxy resin composition containing the curing agent.
The present inventors have made extensive research on the functional group structure and skeletal structure of a compound capable of thermal cure reaction with an epoxy resin, and have consequently found that an epoxy resin composition comprising a compound having a specific functional group structure and a specific skeletal structure can meet the above-mentioned purposes.
According to this invention, there are provided an aryl ester compound composed of a polyhydric phenol in which at least one OH group has been esterified with an organic acid or its derivatives having 1 to 20 carbon atoms, said polyhydric phenol being the condensation product of a substituted or unsubstituted resorcinol represented by the general formula (1): 
wherein each P represents independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms or an aralkyl group having 7 to 20 carbon atoms and i represents an integer of 0 to 2, with a carbonyl compound represented by the general formula (2): 
wherein each of X and Xxe2x80x2 represents independently a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms or an aralkyl group having 7 to 20 carbon atoms, provided that X and Xxe2x80x2 may form a ring together with the carbon atom to which X and Xxe2x80x2 are attached;
a process for producing said aryl ester compound which comprises condensing the substituted or unsubstituted resorcinol represented by the general formula (1) with the carbonyl compound represented by the general formula (2) in the presence of an acid catalyst and then esterified the condensation product thus obtained with an organic acid or its derivatives having 1 to 20 carbon atoms in the presence of a basic compound; an epoxy resin composition which comprises as the essential components:
(A) an epoxy resin,
(B) an aryl ester compound composed of a polyhydric phenol in which at least one OH group has been esterified with an organic acid or its derivatives having 1 to 20 carbon atoms, said polyhydric phenol being a condensation product of a phenolic compound represented by the general formula (4): 
xe2x80x83wherein P and i are the same as defined above and Q represents a hydrogen atom or a hydroxyl group, with the carbonyl compound represented by the general formula (2), and
(C) a cure accelerator; and
a copper-clad laminate obtained by thermoforming a copper foil and a prepreg obtained by impregnating a substrate with the above epoxy resin composition or a solution of the above epoxy resin composition in an organic solvent.
The aryl ester compound of this invention and the component (B) of the epoxy resin composition of this invention are preferably aryl ester compounds represented by the general formula (3): 
wherein n represents the average number of the repeated units and is a value of 1 to 20; each P represents independently a hydrogen atom, a halogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms or an aralkyl group having 7 to 20 carbon atoms; i represents an integer of 0 to 2; each of R1, R2, R3, R4 and R5 represents independently a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, a cycloalkyl group having 5 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms or an aralkyl group having 7 to 20 carbon atoms, provided that R1 and R2 may form a ring together with the carbon atom to which R1 and R2 are attached or R4 and R5 may form a ring together with the carbon atoms to which R4 and R5 are attached; each Z represents independently a hydrogen atom or an acyl group having 1 to 20 carbon atoms, provided that the case where all Z groups are hydrogen atoms is excluded.
In the general formula (3), Z""s may be acyl groups in (n+2) places on average per one molecule (in this case, the esterification percentage is taken as 100%); however, Z""s are such that the esterification percentage may be any value in other cases than where all Z groups are hydrogen atoms (in this case, the esterification percentage is 0%). In order to achieve the purpose of low dielectric constant and low dielectric loss tangent, the esterification percentage is preferably at least 30%, more preferably at least 50%.
In the general formula (3), n represents the average number of the repeated units and may be a value of 1 to 20. However, in view of operability, the average number is preferably a value of 1 to 10.
The polyhydric phenol compound which is used as the starting material for the aryl ester compound can be obtained in such a known manner that resorcinols are condensed with a carbonyl compound in the presence of an acid. The resorcinols are substituted or unsubstituted resorcinols represented by the above general formula (1).
Here, the resorcinols are such as not to have substituents at the 4- and 6-positions and includes resorcinol; alkyl resorcinols, representatives of which are 2-methylresorcinol, 5-methylresorcinol, 2-propylresorcinol, 2-n-butylresorcinol, 5-isobutylresorcinol, 5-t-butylresorcinol, 5-octylresorcinol, 5-nonylresorcinol, 2,5-dimethylresorcinol, 2,5-diethylresorcinol, 2,5-diisopropylresorcinol, 2-methyl-5-butylresorcinol, 2-methyl-5-nonylresorcinol and the like; cycloalkylresorcinols, representatives of which are 2-cyclopentylresorcinol, 2-cyclohexylresorcinol, 2-cycloheptylresorcinol and the like; arylresorcinols such as 5-phenylresorcinol, 5-naphthylresorcinol and the like; aralkylresorcinols such as 5-benzylresorcinol, 5-phenethylresorcinol and the like; and halogenated resorcinols such as 2-chlororesorcinol, 5-chlororesorcinol, 2,5-dichlororesorcinol, 2-bromoresorcinol, 5-bromoresorcinol, 2,5-dibromoresorcinol, 2-iodoresorcinol, 5-iodoresorcinol, 2,5-diiodoresorcinol and the like.
The carbonyl compound is represented by the above general formula (2). The carbonyl compound includes aldehydes, representatives of which are formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, pentylaldehyde, phenylacetaldehyde, cyclohexylacetaldehyde and the like; and ketones such as acetone, methyl ethyl ketone, diethyl ketone, methyl propyl ketone, methyl isobutyl ketone, cyclohexanone, methylcyclohexanone, cycloheptanone, benzyl phenyl ketone, benzyl methyl ketone, methyl phenethyl ketone, acetophenone, acetonaphthenone, indan-1-one and the like.
The acid catalyst used in the condensation reaction between the resorcinol and the carbonyl compound includes inorganic acids such as hydrochloric acid, sulfuric acid and the like; organic acids such as p-toluenesulfonic acid, benzenesulfonic acid, methanesulfonic acid and the like; solid acids such as acid clay, activated alumina, zeolite and the like; acidic ion exchange resins; and the like. The amount of the acid catalyst is preferably 0.01 to 50% by weight, more preferably 0.5 to 20% by weight, based on the total weight of the resorcinols and carbonyl compound charged as the starting materials.
In the condensation reaction, a known non-reactive organic solvent may be used, and this non-reactive organic solvent includes toluene, xylene, dioxane, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide and the like though these solvents are not critical.
The mole ratio of the resorcinols to the carbonyl compound in the condensation reaction is preferably 0.1:1 to 2.0:1, more preferably 0.3:1 to 1.5:1. When the ratio is outside this range, such a problem is caused that an excess of resorcinol remains or the amount of the remaining OH groups of the product becomes small. The reaction temperature is preferably 0 to 200xc2x0 C., more preferably 20 to 160xc2x0 C. When the temperature is lower than this range, the conversion of condensation reaction becomes low and when the temperature is higher than the range, by-products are yielded in a large amount. The reaction time is preferably 1 to 100 hours, more preferably 2 to 80 hours. When the reaction time is shorter than this range, the reaction becomes incomplete, and even if the reaction is effected for a period longer than this range, the yield is not increased, so that such a long term reaction is not economical.
In the condensation reaction, the water formed by the reaction may or may not be removed from the system. When the water is to be removed, it is sufficient to conduct the reaction using an azeotropically dehydratable solvent such as toluene, xylene or the like and a means such as a Dean-Stark tube capable of allowing the reaction to proceed while removing water from the system or the like. Moreover, in order to accelerate the dehydration, the reaction may be conducted under reduced pressure.
The esterification of the polyhydric phenol compound which is the condensation product is effected by reaction with an organic acid or its derivatives having 1 to 20 carbon atoms in the presence of a basic compound.
The organic acid and its derivatives include aliphatic monocarboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, lauric acid, stearic acid, phenylacetic acid, bromoacetic acid and the like; acid halides and anhydrides of the aliphatic monocarboxylic acids; aromatic monocarboxylic acids such as benzoic acid, methylbenzoic acid, naphthoic acid, biphenylcarboxylic acid and the like; acid halides and anhydrides of the aromatic monocarboxylic acids; alicyclic monocarboxylic acids such as cyclopentanecarboxylic acid, cyclohexanecarboxylic acid, cycloheptanecarboxylic acid and the like; acid halides and anhydrides of the alicyclic monocarboxylic acids; and the like. However, these are not critical.
The basic compound used in the esterification reaction includes inorganic basic compounds such as sodium hydroxide, potassium hydroxide and the like; organic basic compounds such as pyridine, triethylamine, triphenylphosphine, imidazole compounds and the like; etc.
In the esterification reaction, a known organic solvent may be used and examples thereof include toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, dioxane, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide and the like. Toluene, xylene and methyl isobutyl ketone are preferred.
In the esterification reaction, the mole equivalent ratio of the organic acid or the acid halide or anhydride thereof to the OH group of the starting polyhydric phenol compound is preferably 0.3:1 to 2.0:1, more preferably 0.5:1 to 1.5:1. When the mole equivalent ratio is outside this range, such problems are caused that an excess of an organic acid remains and the esterification percentage of the product becomes low. The reaction temperature is preferably 20 to 200xc2x0 C., more preferably 40 to 150xc2x0 C. When the temperature is lower than this range, the conversion of esterification becomes low and when the temperature is higher than the range, by-products are yielded in a large amount. The reaction time is preferably 2 to 50 hours, more preferably 4 to 30 hours. When the reaction time is shorter than this range, the reaction becomes incomplete, and even if the reaction is effected for a period longer than the range, the yield is not increased, so that such a long term reaction is not economical.
When an organic acid per se is used as one of the starting materials in the esterification reaction, the water produced by the reaction may or may not be removed from the system. When the water is to be removed, it is sufficient to effect the reaction using an azeotropically dehydratable solvent such as toluene, xylene, methyl isobutyl ketone or the like and a means such as a Dean-Stark tube or the like capable of allowing the reaction to proceed while removing the water from the system, or the like. Moreover, the reaction may be effected under reduced pressure in order to accelerate the dehydration.
The aryl ester compound which is the component (B) of the epoxy resin composition of this invention includes further aryl ester compounds represented by the general formula (5) as preferable ones: 
wherein m represents the average number of the repeated units and is a value of 1 to 10; each T1 represents independently a hydrogen atom or an aromatic ring-free acyl group having 1 to 20 carbon atoms, provided that the case where all T1, groups are hydrogen atoms is excluded; each T2 represents independently a hydrogen atom, an alkyl group having 1 to 10 carbon atoms or a cycloalkyl group having 5 to 7 carbon atoms; h represents an integer of 1 to 3, provided that when h is 2 or 3, T2""s may be the same as or different from one another in one and the same ring; and each T3 represents independently a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an aryl group having 6 to 20 carbon atoms or an aralkyl group having 7 to 20 carbon atoms.
The aryl ester compound of the general formula (5) can be produced by esterifying a novolak type polyphenol compound as the starting material. The novolak type polyphenol compound used as the starting material is prepared according to a conventional method for producing a novolak type phenol resin, for example, by polycondensing phenols with aldehydes using as a catalyst an inorganic acid such as hydrochloric acid, phosphoric acid, sulfuric acid or the like; an organic acid such as benzenesulfonic acid, toluenesulfonic acid, oxalic acid or the like; or a metal salt such as zinc acetate or the like.
The phenols include phenol; o-, m- and p-isomers of monoalkylphenols such as cresol, n-propylphenol, isopropylphenol, n-butylphenol, isobutylphenol, t-butylphenol, octylphenol, nonylphenol and the like; various isomers of multi-substituted alkylphenols, representatives of which are xylenol, diisopropylphenol, methylbutylphenol, di-t-butylphenol, di-t-amylphenol, 2,3,5-trimethylphenol and the like; and o-, m- and p-isomers of cycloalkylphenols, representatives of which are cyclopentylphenol, cyclohexylphenol, cyclohexylcresol and the like. These may be used alone or in admixture of two or more.
The aldehydes include formaldehyde; alkylaldehydes, representatives of which are acetaldehyde, propionaldehyde, butyraldehyde, pentylaldehyde and the like; arylaldehydes such as benzaldehyde, naphthaldehyde and the like; and aralkylaldehydes such as 2-methyl-3-(iso-propenylphenyl)propionaldehyde, 2-methyl-3-(t-butylphenyl)propionaldehyde and the like.
In the general formula (5), T1""s may be acyl groups in (m+1) places on average per one molecule (in this case, the acylation percentage is taken as 100%), but T1""s are such that the acylation percentage is any value in other cases than where all T1""s are hydrogen atoms (in this case, the acylation percentage is 0%). In order to achieve the purpose of low dielectric constant and low dielectric loss tangent in this invention, the acylation percentage is preferably at least 30%, more preferably at least 50%.
In the general formula (5), the average number (m) of the repeated units is 1 to 10, preferably 1 to 5. When the average number of the repeated units is more than 10, the viscosity of the whole system combined with the epoxy resin is increased, so that the workability in the preparation of a laminate is deteriorated. When the average number of the repeated units is less than 1, the cure reaction with the epoxy resin is incomplete, or even if cure reaction is possible, the possibility that the heat resistance of the cured product is remarkably deteriorated is high.
The esterification of the novolak polyphenol compound which is used as the starting material for obtaining the aryl ester compound of the general formula (5) can be conducted in the same manner as in the esterification for obtaining the compound of the general formula (3); however, the organic acid or its derivatives is limited to an aromatic ring-free organic acid having 1 to 20 carbon atoms or an acid halide or anhydride thereof.
The epoxy resin used as the component (A) in this invention means a known compound having at least two epoxy groups in one molecule, and the chemical structure of the compound is not critical. Examples of the compound include difunctional type epoxy compounds such as diglycidyl ether of bisphenol A, diglycidyl ether of tetrabromobisphenol A; trifunctional epoxy compounds such as glycidyl ether of tris(4-hydroxyphenyl)methane and glycidyl ether of 1,1,1-tris(4-hydroxyphenyl)ethane; polyfunctional type epoxy compounds such as glycidyl ether of phenol novolak, glycidyl ether of cresol novolak, glycidyl ether of a novolak obtained by dehydration-condensation of phenols with hydroxyarylaldehydes, glycidyl ether of poly(4-hydroxystyrene), glycidyl ether of phenol-modified polybutadiene, glycidyl ether of phenol-dicyclopentadiene adduct and glycidyl ether of bisphenol A novolak; mixtures of at least two epoxy resins; and the like.
The epoxy resin (A) is preferably an epoxy compound represented by the general formula (6): 
wherein g represents the average number of the repeated units and is a value of 1 to 10; each T4 represents a hydrogen atom or an alkyl group having 1 to 10 carbon atoms; each T5 represents an alkyl group having 1 to 10 carbon atoms or a cycloalkyl group having 5 to 10 carbon atoms, provided that at least one T5 is an alkyl group having 4 to 10 carbon atoms or a cycloalkyl group having 5 to 7 carbon atoms in one and the same ring; j represents an integer of 1 to 3, provided that when j is 2 or 3, T5""s may be the same as or different from one another in one and the same ring.
The epoxy compound of the general formula (6) can be synthesized by such a known method as dehydrohalogenation of a novolak type polyphenol compound and epihalohydrin with a base.
The novolak type polyphenol compound which is used as the starting material for the epoxy compound of the general formula (6) is produced, for example, by polycondensation of substituted phenols with aldehydes using as a catalyst an inorganic acid such as hydrochloric acid, phosphoric acid, sulfuric acid or the like; an organic acid such as benzenesulfonic acid, toluenesulfonic acid, oxalic acid or the like; or a metal salt such as zinc acetate or the like.
The substituted phenols mean monophenols having at least one alkyl group having 4 to 10 carbon atoms or at least one cycloalkyl group having 5 to 7 carbon atoms in one molecule and includes specifically o-, m- and p-isomers of alkylphenols, representatives of which are n-butylphenol, isobutylphenol, t-butylphenol, octylphenol, nonylphenol, methylbutylphenol, di-t-butylphenol, di-t-amylphenol and the like; and o-, m- and p-isomers of cycloalkylphenols, representatives of which are cyclopentylphenol, cyclohexylphenol, cyclohexylcresol and the like; and these may be used alone or in admixture of two or more.
Examples of the aldehydes include alkylaldehydes, representatives of which are formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, pentylaldehyde and the like.
In the general formula (6), the average number (g) of the repeated units is 1 to 10, preferably 1 to 5 on average. When the average number (g) of the repeated units is more than 10, the viscosity as an epoxy resin increases, so that the workability in the preparation of a laminate is deteriorated. Moreover, when the average number (g) of the repeated units is less than 1 the cure reaction is remarkably deteriorated.
Examples of the epoxy compound of the general formula (6) include glycidyl ether of 2-t-butyl-5-methylphenol novolak, glycidyl ether of cyclohexylphenol novolak, glycidyl ether of octylphenol novolak, diglycidyl ether of 1,1-(4-hydroxy-5-t-butyl-2-methylphenyl)butane, and the like.
The epoxy resin (A) of this invention may be an epoxy resin obtained by previously reacting an epoxy resin with a halogen-containing bisphenol compound represented by the general formula (7): 
wherein T6 represents a hydrogen atom or a methyl group and two T6""s may be the same as or different from each other; Y and Yxe2x80x2 represent halogen atoms and may be the same as or different from each other; and each of k and l is independently an integer of 1 to 4.
The halogen-containing bisphenol compound of the general formula (7) includes specifically tetrabromobisphenol A, tetrachlorobisphenol A, tetraiodobisphenol A, tetrabromobisphenol F, tetrachlorobisphenol F and the like, and tetrabromobisphenol A is preferably used from the viewpoint of economy and efficient flame-retardation.
The reaction of the epoxy resin with the halogen-containing bisphenol compound of the general formula (7) in this invention may be conducted in a manner known per se.
For example, the above components can be allowed to react in the presence of a basic catalyst such as triphenylphosphine, imidazole or the like. This reaction makes it possible to control the glass transition temperature based on a change in distance between cross-linking sites and to impart a flame-retardance due to a halogen-containing compound without being accompanied by volatilization of low molecular weight materials during curing.
The above two components may be used in any proportion, and it is preferable to mix the two components so that the proportion of the OH groups in the halogen-containing bisphenol compound becomes 0.05 to 0.75 mole per 1 mole of the epoxy group in the epoxy resin and then subjecting them to reaction.
Moreover, it is preferable to mix the epoxy resin (A) with the aryl ester compound (B) so that the ratio of the number of moles of the acyl groups in the aryl ester compound to the number of moles of the epoxy groups in the epoxy resin becomes 0.3:1 to 1.5:1, and this ratio is more preferably 0.5:1 to 1.2:1. When said ratio is outside this range, a failure of curing is caused and good cured product is not obtained.
The cure accelerator in this invention is a conventional compound capable of accelerating the cure reaction between the epoxy resin and the curing agent and examples thereof include imidazoles such as 2-ethyl-4-methylimidazole and 4-methylimidazole; tertiary amines such as triethylamine, benzyldimethylamine and 1,4-diaza-bicyclo [2.2.2]undecene; quaternary ammonium salts such as tetra-n-butylammonium bromide and tetra-n-amylammonium bromide; phosphorus compounds such as triphenylphosphine; and the like. The proportion of the cure accelerator is preferably 0.05 to 3% by weight based on the weight of the resin composition.
It is also possible to use other thermosetting resins than the epoxy resin or a thermoplastic resin having a functional group, in such a proportion that the effect of this invention is not damaged. Specifically, a cyanate resin, a maleimide resin, a glycidyl-modified polybutadiene, maleic anhydride-modified polyethylene or the like may be used.
In this invention, known additives such as flame retardant, surface-treating agent and the like may be added to the epoxy resin composition depending upon the purposes.
The flame retardant includes antimony trioxide, aluminum hydroxide, red phosphorus and the like, and the surface-treating agent includes a silane-coupling agent.
The preparation of the copper-clad laminate of this invention can be conducted according to a known method. That is to say, a substrate is impregnated with the above epoxy resin composition or with a resin varnish prepared by dissolving the above epoxy resin composition in an organic solvent, the impregnated substrate is heat-treated to prepare a prepreg, a copper foil is placed on the prepreg and the resulting assembly is thermoformed to prepare a copper-clad laminate.
The organic solvent used includes acetone, methyl ethyl ketone, methyl isobutyl ketone, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, toluene, xylene, N,N-dimethylformamide, dioxane, tetrahydrofuran and the like, and these solvents may be used alone or in admixture of two or more.
The substrate to be impregnated with the epoxy resin composition or a varnish of the epoxy resin composition includes woven fabrics, non-woven fabrics and mats composed of inorganic or organic fibers such as glass fibers, polyester fibers, polyamide fibers and the like; paper; and the like, and these may be used alone or in combination.
The conditions for the heat-treatment for obtaining the prepreg may be varied depending upon the kind and amount of the solvent, the catalyst added and the various additives; however, it is preferable to conduct the heat-treatment at a temperature of 80xc2x0 to 220xc2x0 C. for a period of 3 to 30 minutes.
The thermoforming is conducted, for example, by heat-press molding at a temperature of 150xc2x0 to 300xc2x0 C. at a molding pressure of 10 to 100 kg/cm2 for a period of 20 to 300 minutes.
Examples of this invention are shown below; however, this invention should not be construed to be limited thereto. In the Examples, the epoxy equivalent is defined as the molecular weight of the epoxy resin per one epoxy group, and the OH equivalent is defined as the molecular weight of the polyhydric phenol compound per one OH group.